Carbon-13 chemical shielding tensors in alkanedicarboxylic acids. Influence of molecular geometry on the carboxyl carbon tensors in alkanedicarboxylic acids and related compounds

The principal elements of 13C chemical shift tensors in some dicarboxylic acids were determined from powder samples by two methods: (i) from the theoretical fit of the experimental data and (ii) from the side-band intensities of the CP-MAS spectra using the graphical procedure of Herzfeld and Berger. Neither the methylene nor the carboxyl carbons are axially symmetrical. The methylene tensors corresponding to higher dicarboxylic acids were not analysed owing to severe overlap of the different carbon tensors. The influence of local molecular geometry on the carbon 13C chemical shielding tensors in dicarboxylic acids and other related organic molecules are also reported. The σ22 component of the tensor shows a linear correlation as a function of the structural parameter, the OCO bond angle, around this carboxyl carbon atom. The significance of the symmetry factor calculated for the carboxyl carbons is also presented